Participants

Abstract: Packing particles on a curved surfaces is a fundamental problem in Soft Matter to explore how ordered materials interact with geometry and with applications as diverse as Pickering Emulsions and cryptography. Here, we study packings of bidispersed spherical particles on a spherical surface. The presence of curvature necessitates defects even for monodispersed particles; bidispersity either leads to a more disordered packing for nearly equal radii, or a higher fill fraction when the smaller particles are accomodated in the interstices of the larger spheres. Variation in the packing fraction is explained by a percolation transition, as chains of defects or scars previously discovered in the monodispersed case grow and eventually disconnect the neighbor graph. This new result connects the field of crystallography on curved surfaces to glasses and amorphous materials.

Abstract: Optical tweezers are a useful tool in cellular biology, with the ability to noninvasively manipulate microscopic particles, and perform force measurements. Here I use Brownian dynamics simulations corresponding to using optical tweezers to hold and detach a particle from a membrane. However, the rate at which a particle is pulled out of an optical trap affects the trajectory by which the particle escapes from the trap. In this talk I shall discuss how, for a finite pulling rate, the path with which the particle detaches varies for different velocities. Likewise, the work done to detach a particle from a membrane varies depending on the escape velocity. I shall describe my simulation approach, using Crooks fluctuation theorem and Jarzynski equality, to extract a membrane potential. These simulations allow the use of optical tweezers to measure a membrane potential based on its attachment to a particle.

Abstract: I will describe how to efficiently implement connectivity changing moves which allow for rapid Monte Carlo sampling of dense polymer configurations, such as Hamiltonian paths. I will then give an overview of some important features of dense polymers, such as the occurrence of long-range correlations within the chains and the effect of entanglement on dynamics, and explain how efficient Monte Carlo algorithms will help us to understand them.

Abstract: Coinciding with the publication of our book on non-equilibrium molecular dynamics, we have made available a simple computer program for carrying out equilibrium and non-equilibrium molecular dynamics simulations to compute transport coefficients for a limited set of simple molecular models, mainly for pedagogical purposes. In this talk, I will describe the capabilities of this program and display some results that can be obtained with it.

Abstract: Most of us have tried this at home: smash cereal with a spoon. Yet, the science of snap, crackle and pop extends way beyond a cereal ad. In fact, using such a simple experiment we reveal surprisingly rich compaction patterns due to competing processes of internal collapse and recovery. Using a simple spring-lattice model that captures these two processes, we successfully explain previously observed patterns in cereal, snow and sandstones. Subsequently, we use the model to guide us in the discovery of novel patterns, which we confirm experimentally in cereal. A further set of experiments with cereal partially soaked by fluid (milk/water) under constant pressure reveals coherent rice-quakes that could be explained by coupled fluid diffusion and the chemical degradation of the solid matrix; similar conditions often prevail in rockfill dams, which frequently fail unexpectedly. Our work reveals bifurcation in solids reminiscent of critical phenomena near phase transitions, and thus will be of relevance for people interested in soft matter physics, complex systems, and non-equilibrium thermodynamics.

Abstract: The phase diagram of a Lennard-Jones fluid is similar to the one of the Ising ferromagnetic: both have a critical point, which separates the ordered phase from the disordered one. The analogy between a fluid and a ferromagnetic system has been very useful in describing critical phenomena, employing universality of the second-order phase transition. This analogy, however, is not limited to the region of small vicinity of the critical point. Comparing the topological structure of the phase diagrams of a fluid and a ferromagnetic one can conclude that there exists an isomorphism between them. This is the case in a broad interval of temperatures along the phase coexistence, and is valid both in 2D and 3D. This so-called Global Isomorphism can be used to derive properties of a complex liqud-vapor interface using much simpler lattice gas approach of the Ising model. In particular, in this work we demonstrate this for the surface tension, deriving its expression for a fluid without any fitting parameter.

Abstract: We propose a stochastic parametrization strategy for slow-fast systems with finite time-scale separation. The method can be equally be applied to deterministic and stochastic multi-scale systems and is based on Edgeworth approximations. The stochastic parametrization consists of a surrogate systems the parameters of which are matched to produce the same Edgeworth expansions up to any desired order of the original multi-scale system. It incorporates deviations from the standard homogenization limit which is valid for infinite time-scale separation. We corroborate our analytical findings by numerical examples.This is joint work with Jeroen Wouters

Abstract: Many gels and yield-stress fluids which find widespread use in consumer products as texture and viscosity modifiers owe their mechanical properties to fibrous microstructures. While the rheological implications of these additives are understood, the effect that they have on particle transport within these fluids are not. In this work particle diffusion through fibrous gel networks is simulated and the effective diffusion coefficient of differently sized particles predicted. The results from these simulations have implications for the interpretation of data from microrheology experiments, in particular the mean-square-displacement (MSD) plots produced.

Abstract: The problem of forces experienced by an object moving in a granular material has many practical applications, and is of fundamental interest in fluid mechanics. This classical configuration has been extensively studied in Newtonian or non-Newtonian fluids, and allows to probe the rheological properties of the material. Here we study experimentally and numerically the forces experienced by a horizontal cylinder rotating around the vertical axis in a granular material. We show that the cylinder experiences a strong lift force in this configuration, proportional to its buoyancy with a high amplification factor. This effect is due to the specific properties of the granular medium, the presence of a pressure gradient modifying completely the flow around the object. A second striking observation is made after several rotations of the cylinder. The drag force dramatically drops and becomes independent of depth, showing that the object no longer perceive the weight of the grains above it. The rotation of the cylinder creates a strong stress anisotropy, that screens the weight of the grains and creates a low pressure bubble above the object.

Abstract: The binding of molecules in solution is a fundamental process in soft matter. How much entropy molecules lose upon binding, however, has proved controversial for over fifty years [1,2]. We show that differences in the entropy of binding arise because of the different ways that entropy can be decomposed between molecules in solution [3]. The pseudo-gas-phase Sackur-Tetrode model for a solute yields greater losses in translational and rotational entropy than the cell model. Moreover, the models have qualitative differences in their dependence on the sizes of the molecules involved. Such issues are also expected to influence the corresponding change in internal entropy [4].

Abstract: We use molecular dynamics simulations to study the dynamics of Janus particles, micro- or nanoparticles that are not spherically symmetric, in the flow of simple liquids. In particular, we consider spheres with an asymmetry in the solid-liquid interaction over their surfaces and calculate the forces and torques experienced by the particles as a function of their orientation with respect to the flow. We also examine particles that are deformed slightly from a spherical shape. We compare the simulation results to the predictions of a previously introduced theoretical approach, which computes the forces and torques on particles with variable slip lengths or aspherical deformations that are much smaller than the particle radius. We find that there is good agreement between the forces and torques computed from our simulations and the theoretical predictions, when the slip condition is applied to the first layer of liquid molecules adjacent to the surface. The results allow us to estimate critical flow rates at which Janus clusters will become unstable.

Abstract: Configurational search or exploration in three dimensions can be sped up by adding additional dimensions that allow bond frustrations, knots, or chain crossings to freely occur. This removes common bottlenecks in the relaxation kinetics of clusters and polymer chains. However, these higher dimensional configurations must eventually be mapped back to reality. In this presentation I will discuss strategies, obstacles, and benefits of this approach, and show how we used it to aid cluster search, and protein folding simulations.

Soon-Hwan Hwang

ED&C

Ahmad Jabbarzadeh

The University of Sydney

Ravi Jagadeeshan

Monash University

Size, shape and diffusivity of a single Debye-Hückel polyelectrolyte chain in solution

Abstract: Impact performance of intrinsically brittle materials at the nanoscale is a topic of growing interest, for instance, in vacuum cold spray of nanomaterials. The work presented here is the preliminary work where the simulation framework is built and tested for the impact of titanium particles on a titanium substrate. The work is an examination of single crystalline titanium nanoparticle deposition upon another titanium substrate with molecular dynamics simulation (MD) technique. Realistic potentials are used to allow for the deformation of the substrate and rigid nano-particle upon impact, the main concept of research is to assess deformation of particles and substrate during a modified cold spray process the so-called Aerosol Deposition Method (AMD), based on critical velocity of particles, size of particles and other critical parameters of AMD. Other important parameters in a deposition process such as local stresses, strain and temperature at the substrate/particle impact region are calculated under practical conditions of vacuum cold spray process. Particle deformation and bonding condition have been simulated using LAMMPS molecular dynamic simulator. Further experiments are planned in order to compare MD simulation results with Aerosol Deposition process parameters.

Abstract: Knudsen flows can arise in applications of microporous materials, such as filtration, when external conditions induce low-density transport. Over a decade ago, we proposed a model to predict the collective diffusion of Knudsen flows in uniform micropores (such as some microporous carbons and silicas). This model works in the zero-density (high Knudsen-number) limit, and has proven popular as a means of estimating Knudsen transport in this limit, and as a basis for developing density-dependent models. Most of these density-dependent models extend the zero-density model using approaches that are very different in philosophy to the original model. In this presentation, I will show how the original model can be adapted to include density dependence, and compare its predictions with existing data.

Abstract: Nanomechanical devices operated in ambient conditions generate low Mach number, non-equilibrium disturbances in the surrounding gas. A range of numerical methods are available for solving the Boltzmann equation in order to simulate these flows. For high Knudsen number flows, stochastic Monte Carlo methods offer an effective approach, while for near continuum flows, a range of deterministic methods provide superior efficiency. In this talk, we propose a hybrid approach which combines features of several these techniques, providing good performance across the entire range of rarefaction.

Abstract: We will present some new bond-orientational order (BOO) parameters that can be measured in the projection geometry with small probe transmission diffraction. Such a set of order parameters will be extremely useful for disordered, glassy and liquid systems in which the particle positions in three-dimensions cannot be directly measured, for example atomic systems, small nanoparticles and biomolecules. Tuning the incident radiation allows the same methodology to be employed for disordered systems across many decades in length scale. We will demonstrate the correspondance between the “projected” BOO parameters to the traditional three-dimensional ones and discuss how we can use the new parameters to fingerprint local symmetry and quantify variability in local structure. Such parameters accessed via transmission diffraction show promise for understanding and mapping local structure and probing structural change during dynamic processes.

Abstract: A concentration gradient along a fluid-fluid interface or a solid-fluid interface can cause flow. On a microscopic level, this so-called Marangoni effect or diffusio-osmosis can be viewed as being caused by a gradient in the pressures acting on the fluid elements, or as the chemical-potential gradients acting on the excess densities of different species at the interface. Here we compare the results of direct non-equilibrium Molecular Dynamics simulations with the flows that would be generated by pressure and chemical potential gradients. We find that the approach based on the chemical potential gradients agrees with the direct simulations, whereas the calculations based on the pressure gradients do not.

Abstract: Granular media -- including familiar systems such as sand, soil, rice, glass beads -- are, ironically, considered singularly unusual by many physicists: They violate energy conservation, thermodynamics, the fluctuation-dissipation theorem, the Onsager relation and more general principles. They are, in one word, athermal. In this talk, it will be argued that, properly viewed, granular media are quite normal and thermal. The usual tools of theoretical physics and statistical mechanics are therefore applicable and useful, and have been employed to derive a set of partial differential equations: -- GSH (for granular solid hydrodynamics) -- by appropriately generalizing the Navier-Stokes equations. The result is capable of accounting for a wide range of granular phenomena, including static stress distribution, elasto-plastic motion, the critical state, the $\mu(I)$- and Kamrin's nonlocal-rheology, shear band, compaction, and sound propagation.

Abstract: When granular materials flow, the constituent particles segregate by size and align by shape. The impacts of these changes in fabric on the flow itself are not well understood, and thus novel non-invasive means are needed to observe the interior of the material. Here, we propose a new experimental technique using dynamic X-ray radiography to make such measurements possible. The technique is based on Fourier transformation to extract spatiotemporal fields of internal particle size and shape orientation distributions during flow, in addition to complementary measurements of velocity fields through image correlation. We show X-ray radiography captures the bulk flow properties, in contrastto optical methods which typically measure flow within boundary layers, as these are adjacent to any walls. Our results reveal the rich dynamic alignment of particles with respect to streamlines in the bulk during silo discharge, the understanding of which is critical to preventing destructive instabilities and undesirable clogging. The ideas shown here are directly applicable to many other open questions in granular and soft matter systems, such as the evolution of size and shape distributions in foams and biological materials.

Abstract: We present a new analysis method that inverts electron or x-ray diffraction data of disordered materials into a real-space angular distribution function. Technically, this is a 3D three- and four-body correlation function that generalises the 1D pair-distribution analysis common in conventional small- and wide-angle x-ray scattering methods. In liquids and glasses, the new analysis method could be used to measure bond-angle distributions and gain greater sensitivity to medium-range order. We discuss the prospects for using the new analysis method to study amorphous materials, soft matter, colloids and protein conformations with electron diffraction, synchrotrons and x-ray free-electron lasers.

Abstract: The Coleman-Markovitz equation, derived in 1964 following the retarded motion expansion for a second order fluid, has long been a well accepted and well used result in Rheology. However, following the development of linear and non-linear response theory no attempts at a derivation from a statistical mechanics point of view have been made. Here we present one approach towards the derivation of the Coleman-Markovitz equation by substituting the SLLOD equations of motion into the Liouvillian and applying non-linear response theory.

Abstract: Recent simulation and experimental results indicate that the solution stability and assembly of metal and semiconductor nanoparticles are strongly dependent on how passivating molecules order on their surface. Our molecular dynamics simulations on spherical nanoparticles passivated with alkylthiol chains indicate that the temperature of the ligand order-disorder transition increases with core size and ligand length. This behavior is consistent with previous experimental observations and is related to the larger gain in entropy associated with the disordering of ligands on smaller particles or with shorter chains. The transition temperature also increases as the solvent gets more similar to the ligands, which is in direct contradiction to what would be expected according to classical methods for evaluating colloidal stability such as Flory-Huggins solution theory. These trends indicate that a wide range of parameters can affect the structure of the ligand shell, and sometimes in unexpected ways.

Abstract: Molecular crystals can take on many different forms with different orientational and positional relationships, making it difficult to identify crystal structures when they do arise. I present my work developing a generalisable technique for the characterisation of local structure.

Abstract: Material properties are an important component of nanotechnology when considering the functionality and robustness of new devices. What are some of the extreme cases that we could expect when looking at the melting and other thermally driven instabilities in metal nanowires? We consider the melting process for two different materials based on some known material properties, Nickel and Aluminium. Phenomenological models and molecular dynamics simulations are used to investigate the melting process and dynamics of the two materials. The simulated data is compared to the developed phenomenological models.

Pierre Rognon

Particles and Grains Laboratory, School of Civil Engineering The University of Sydney

Abstract: Despite the latest improvements to synthesise a great variety of nanoparticles with complex aspherical shapes which revealed a wide range of new phases in colloidal science, many aspects of shape and their effect on the micro-scale can still only be investigated by using computational simulations.

​​​Also tapered ellipsoids, reminiscent of “pear-shaped'' particles, have not been realised in experiments yet. Previous computational studies based on hard Gaussian overlap functions showed, however, that these pears form bicontinuous cubic gyroid phases which are famously found in various biological lipid bilayer systems.

Based on these results we probe an entropic mechanism which enables tapered particles in a mixture dominated by small spheres to spontaneously assemble into micellar structures. Therefore, we perform Monte Carlo simulations with two different tapered particles - pears and tapered spherocylinders - and compare their capability to form micelle clusters. In addition we investigate systems where the micelles themselves form bigger, potentially cubic phases in a hierarchical fashion. From this we expect to get a deeper understanding in the formation of bicontinuous structures in nature and a notion of the similarities and differences between enthalpic and entropic self-assembly.

Alexander Smith

University of Auckland

Droplet Motion on Superhydrophobic Surfaces

Patrick Spicer

UNSW Sydney

Gang Sun

University of Sydney

The Structural Origin of Enhanced Dynamics at the Surface of a Glassy Alloy

Abstract: Although metallic glasses are well known for strain softening, transient hardening has been evidenced by serrated flow stress and post-relaxation stress overshoot. By simulating cyclic unloading-reloading and shear band ‘slide-stop-slide’ process of metallic glass Cu50Zr50 , we found that free volume in the shear band changes bifurcately while local fivefold symmetry increases consistently during compressive and tensile relaxation. We propose that the relaxation of structural symmetry, instead of free volume, in the shear band is the atomistic mechanism for transient hardening. An apparent softening phenomenon was also observed when the sample is unloaded from the early or intermediate stage of shear band propagation. This softening results from the inertia of band propagation and is possibly related to material fatigue under cyclic elastic loads. We did not directly observe stress serrations via molecular dynamics simulations due to the very high simulated strain rates. While athermal quasistatic simulations produce serrated flow stress, we note that such serrations result from global avalanches of shear events rather than transient hardening of shear bands.

Abstract: There have been extensive efforts to assemble nanorods into compact monolayers with the rods aligned perpendicular to a substrate or to the plane of a membrane, due to the potential use of such structures in water purification, sensing and solar energy capture. Such assembly, however, has proved difficult to achieve reproducibly as the monolayer structure can often only be accessed kinetically. The synthesis of patchy rods, however, has opened up new possibilities for tuning assembly. Similar to Janus particles, such rods have one or both ends that differ chemically from the rest of the particle. In this work, we use Monte Carlo simulations to characterise the phase behaviour of patchy rods in the presence of an interface and show that a subtle balance between rod-rod and rod-substrate interactions determines whether nucleation and growth of assemblies occurs with the rods oriented parallel or perpendicular to the interface. We find that both surface smectic and crystal phases can form, and that formation of the perpendicular monolayer structure is thermodynamically stable for these particles over a wide range of parameter space, including changes in the rod aspect ratio and the length- and orientation-dependence of the rod-rod and rod-substrate interactions.

Abstract: Colloidal nanorods made of semiconductors or noble metals, exhibit useful and tunable properties that depend on their size, and also on how they are ordered on larger scales. One way to cause nanorods to assemble into an ordered structure is the addition of a polymer depletant. We seek to understand the initial nucleation of these structures, and factors that affect the final structure forme.